The present invention relates to a method of erasing dots in inkjet printers (plotters). More particularly, it relates to a pixel array printing for a thinning processing of dots in a high density printing region.
In the inkjet printers (plotters), a plotting is applied by spraying ink dots in a variety of media. Depending on kinds of the media, the ink tends to stain or is almost not absorbed which permits overflow on the surface that spoils a quality of image or picture greatly. Although this problem might be solved by minimizing a quantity of discharged ink of the inkjet head, actually, inkjet heads having a high tenacity to materialize completely the ink discharge quantity that meets with each media have not been introduced into a market. Whereas a method of reducing an ink density by not permitting the discharge of dots that are produced from the discharge of the ink has been considered. For this method, some of bits of 1 (on) to the bit map data in the plotter controller may be converted to 0 (off). This processing is called the thinning processing, and it is materialized with a firmware in the plotter controller.
The thinning processing is a processing that converts some of bits of 1 of the bit map expanded in the plotter controller to 0 as described in the foregoing. For example, as shown in FIG. 6, in order to change an original picture with 100% density to a picture with 80% density, 20 pieces of dots per 1 dot (black in the drawing) of 100 pieces are replaced to 0 (white in the drawing). More concretely explained, a conjunction operation with the bit map pattern having 80% density is conducted. If the conversion of 0 with concentration in a small region is performed, a density irregularity appears that reduces a quality of picture so that the rewriting from 1 to 0 is performed by dispersing it over a wide range as much as possible. As explained in the foregoing, the case where an original picture with 100% density changes to the 80% density as a result of the thinning processing is called a thinning rate 80%.
In the foregoing thinning processing, the ink density might be lowered, but problems such as disappearance of thin lines, deterioration of shape, or disappearance of a picture with low density occur.
[Problems of disappearance of thin lines]
An extreme example of the disappearance of thin lines is shown in FIG. 14. There are thick lines and thin lines with 100% density in an original picture (a). With respect to this picture, when the density is lowered to 50%, there is a great apprehension that the thin lines like (b) disappears completely. Although the thick lines have not lost the shape, there is a possibility that the shape is deteriorated that lowers a quality of picture depending on a position or angle of a segment of a line.
[Problem of disappearance of original picture with low density]
For example, as shown in FIG. 15, in case a 50% thinning is applied to two pieces of a segment of a line with gray scales 100% and 25%, there is a possibility that an original picture with 25% gray scale disappears. In this case, the disapperance occurs as it is entirely dependent on a position of a segment of a line or a pattern of gray scale or the density is not lowered at all. The reason for a great influence of this phenomenon on the quality of picture is obvious in the case of the thinning processing that relatively and greatly lowers the density relative to the gray scale data of low density.
The cause of loosing the thin lines and low density picture data resides in a point that the dots are erased which are already a few in a region. In an extreme case, in case the thinning processing is applied to the data with 1 dot only, results are such that no change occurs or the dot is erased thereby producing a white paper. A preferable object of the thinning processing is a portion with a high dot density to a certain degree. For example, in case a thinning processing, namely, a conjunction operation with a thinning pattern is applied to reduce the density to 75%, if the thinning processing is applied to a point having higher than 75% density upon an examination of a neighborhood of dots of the original picture that becomes its object, the picture loss phenomenon can be restrained.
However, in this method, the picture with 75% density becomes the picture with 56% density that is 75% of 75%, and the picture with less than 74% remains as the original picture.
As a result, the reversal of the density occurs, which produces a possibility of losing the gradation of the original picture. As a special example of this method to show the fact, in the official gazzette of Japanese Patent Laid Open Publication No. Hei 7-236057, a method of thinning operation against the density portion of 100% is proposed, but the method only executes the reduction of the density of a full single color picture, and maintains the picture density as it is against the high density picture of about 90%, and fails to achieve the original object of the invention that reduces the ink discharge amount.
An object of the present invention is to prevent this gradation reversal phenomenon.